The field of the invention pertains to the optical measurement of angles and straightness for a variety of scientific and industrial purposes.
Optical measurement of angles is conventionally performed using either an interferometer or an autocollimator. Straightness errors are conventionally measured by dial gages and straightedges. More advanced methods use optical straightedges, laser interferometers or laser alignment techniques with quadrant diodes or position-sensing detectors (PSDs). The interferometer is based on the principle of interference of two laser beams which traverse slightly different optical paths whenever an angular displacement occurs. Various configurations have been developed and investigated for improving the accuracy and simplifying the structure. Among them, HP5528A interferometer measurement system from Hewlett-Packard Corporation is probably the most reliable and widely used instrument, especially in the calibration and accuracy maintenance of machine tools and Coordinate Measuring Machines (CMMs). But, this instrument only offers the capability of measuring one error component at a time.
An autocollimator measures an angular displacement by detecting the lateral displacement of a laser beam reflected from a mirror subjected to the angular displacement. Traditional autocollimators with manual operation have long been used for alignment. Their accuracy is usually around one second of arc. More recent laser opto-electronic autocollimators show higher accuracy and are used for surface profiling of optical elements, geometric error measurement of machine tools, and laser mirror servocontrols. Although both methods provide high resolution, devices based on these principles are usually large, making the devices hard to integrate with machines for on-line measurement.
However satisfying in measurement accuracy, almost all the systems measure only one or two error components at a time. This makes the calibration of an instrument or machine tool rather complicated and time consuming. Furthermore, if such measurements systems are used for actively controlling a linear-motion system, usually multiple separate measurement instruments need to be set up to measure all the error components simultaneously, which greatly complicates the system.
U.S. Pat. No. 4,390,781 discloses a method and apparatus for detecting the focussing error signal of an objective lens. The method and apparatus utilize a polarizing beamsplitter inserted between a collimating lens and an objective lens. The beamsplitter reflects light flux into a detection prism with a reflection surface and thereby to a light detector.
U.S. Pat. No. 4,505,584 discloses further improvements in the method and apparatus of the above patent comprising the application of specific thin films of alternating index of refraction to the prism. The result is improved sensitivity of the detector without appreciable increase in size of the prism.
U.S. Pat. No. 4,930,896 discloses a surface structure measuring apparatus that utilizes a focus detecting system with light deflectors conjugate to the pupil of an objective lens. Pertinent to the new method and apparatus described below is the apparatus shown in FIG. 3 wherein two prisms and two photo-detectors are used as a part of a surface measuring device. The three patents above, however, are generally directed to surface measurement as distinguished from angle measurement.
To make the calibration of instruments and machine tools more efficient and provide for possible real-time error compensation based on on-line measurement, the below disclosed system has been developed to measure multiple error components simultaneously.